Adenovirus genomes are linear, double-stranded DNA molecules of approximately 36 kilobase pairs. Each extremity of the viral genome has a short sequence known as the inverted terminal repeat (or ITR), which is necessary for viral replication. The well-characterized molecular genetics of adenovirus render it an advantageous vector for gene transfer. Portions of the viral genome can be substituted with DNA of foreign origin. In addition, recombinant adenoviruses are structurally stable.
Adenoviruses thus may be employed as delivery vehicles for introducing desired polynucleotide sequences into eukaryotic cells, whereby the adenovirus delivers such polynucleotide sequences to eukaryotic cells by binding cellular receptors.
Adenoviral vectors, however, elicit immune responses, and such immune responses correlate with decreased efficiency of gene transfer and expression after repeated administration. (Yei, et al., Gene Therapy, Vol. 1, pgs. 192-200 (1994)). It also was found that neutralizing antibodies to adenovirus block successful repeat administration of the adenovirus. (Smith et al., Nature Genetics, Vol. 5, pgs. 397-402 (1993); Kozarsky, et al., J. Biol. Chem., Vol. 269, No. 18, pgs. 13695-13702 (May 1994)).
Immunity to adenovirus is type specific (Wadell, "Molecular Epidemiology of Human Adenoviruses," in Current Topics in Microbiology and Immunology, Vol. 110, pgs. 191-220 (1984)), and infection with a particular serotype of adenovirus confers immunity only to that serotype. Successful DNA transduction has been demonstrated using sequential administration of different serotypes. (Mastrangeli, et al., Human Gene Therapy, Vol. 7, pgs. 79-87 (Jan. 1, 1996)). In Mastrangeli, an immunizing dose of wild-type Adenovirus 5 (subgroup C), Adenovirus 4 (subgroup E), or Adenovirus 30 (subgroup D) was administered intratracheally to rats, followed by an intratracheal administration of a replication-deficient subgroup C-derived recombinant adenovirus. Efficient gene transfer was not achieved in the rats that were given Adenovirus S. In contrast, effective gene transfer was achieved in the rats that were given Adenovirus 4 or Adenovirus 30.
Kass-Eisler, et al., Gene Therapy, Vol. 3, pgs. 154-162 (1996) disclose the administration of a vector derived from Adenovirus 5 which includes a chloramphenicol transferase (CAT) gene to one-day-old mice. Sixty days later, the mice received a second dose of the same vector. After the second administration, expression of CAT increased from about 2,900 units at a 57-day time point to about 27,000 units five days after the second administration. Although the expression of CAT increased, increases in the levels of neutralizing antibodies against Adenovirus 5 also were detected. Thus, Kass-Eisler, et al. show that a second injection of adenovirus is possible only if the normal immune response is "circumvented," such as, for example, by administering the first dose to neonatal mice that are incapable of mounting an effective immune response and perhaps become "tolerant" of the injected adenovirus.